To increase the safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel has become a valuable tool. One recent successful forward looking approach is based on the excitation and registration of tunnel surface- aves. For further development and for finding optimal acquisition geometries it is important to study the propagation characteristics of tunnel surface-waves. 3D seismic finite difference modelling and analytic solutions of the wave equation in cylindrical coordinates reveal that at higher frequencies, i.e., if the tunnel-diameter is significantly larger than the wavelength of surface-waves, these surface-waves can be regarded as Rayleigh-waves confined to the tunnel wall and following helical paths along the tunnel axis. For lower frequencies, i.e., when the tunnel surface-wavelength approaches the tunnel-diameter, the propagation characteristics of these surface-waves are similar to S-waves. We define the surface-wave wavelength-to-tunnel diameter ratio w to be a gauge for separating Rayleigh- from S-wave excitation. For w > 1.2 tunnel surface-waves behave like S-waves, i.e. their velocity approaches the S-wave velocity and the particle motion is linear and perpendicular to the ray direction. ... mehrFor w < 0.6 they behave like Rayleigh-waves, i.e., their velocity approaches the Rayleigh-wave velocity and they exhibit elliptical particle motion. For 0.6 < w < 1.2 a mixture of both types is observed. Field data from the Gotthard Base Tunnel (Switzerland) show both types of tunnel surface-waves and S-waves propagating along the tunnel.